#include<cstdio>
#include <iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>

using namespace std;
typedef long long ll;
typedef pair<int, int> Pr;

ll N, K;

int main()
{
	cin >> N >> K;

	if (K == 0 || K > N)
		cout << 0 << endl;
	else {
		if (N % 2 != 0 && N/2+1 == K) {
			cout << N - 1 << endl;
		}
		else
			cout << N - 2 << endl;
	}
	return 0;
}